Determination of large strains in metalforming

Certain techniques for the experimental determination of large strains in metalforming are discussed. The methods employ a grid pattern marked on the surface of the workpiece before forming and subsequently measured after deformation. The grid method is a surface phenomenon and the strain determination is reduced to a two dimensional problem. Any measurements taken on the initial and final grid configurations only, without knowing the deformation path, are insufficient to determine the strains precisely. Nevertheless, in practice, strains are determined by such a technique, often by using grid circles and by further assuming they deform into ellipses. The deformation process which transforms circles to ellipses and straight lines to straight lines, still does not define the straining path. Under such a deformation mode strain paths can be divided into two main types, those where an initially orthogonal pair of line elements (the principal axes) remain orthogonal throughout the deformation and those where they do not. The same change in shape can be achieved by either type of straining path, but the distinction between each straining mode is discussed and analysed in simple terms. The analysis is performed with reference to a square or quadrilateral grid of lines since automatic image analysis can be most effectively exploited when using coordinate or nodal point measures.